'On-the-fly' changing of the type of GnuPG encryption
and/or signing used (MIME/ascii) in the Compose window

Manual selection of MIME type and encoding
for attachments

Save all attachments in a mail at once

Save 2 or more selected files to a single file

SSL certificate management

User-definable newsgroup names abbreviation length

Address book: import from LDIF, Mutt or Pine natively

Address book: import from other mailers via scripts

Auto-completion of recipients (also via LDAP)

Harvest addresses for addressbook

Addressbook export to various formats

Per-folder properties:

Return-receipt

Save messages

Default To:

Subject simplification

Folder CHMOD

Default account

Folder colour

Offline synchronisation

Default dictionary

Plugins

The following plugins are included in the source package.

Bogofilter

Enables the scanning of incoming mail received from a POP,
IMAP, or LOCAL
account using Bogofilter. It can optionally delete mail
identified as spam or save it to a designated folder, and also
can be used to train Bogofilter.
Bogofilter is available from
http://www.bogofilter.org/

Dillo HTML Viewer

Enables the viewing of html messages using the Dillo web
browser, version 0.7.0 or newer. It uses Dillo's --local
option by default for safe browsing. Dillo is available from
http://www.dillo.org

Handles PGP/Inline signed and/or encrypted mails. You can
decrypt mails, verify signatures or sign and encrypt your
own mails. Requires GnuPG and GPGME:http://www.gnupg.org/gpgme.html

PGP/MIME

Handles PGP/MIME signed and/or encrypted mails. You can
decrypt mails, verify signatures or sign and encrypt your
own mails. Requires GnuPG and GPGME:http://www.gnupg.org/gpgme.html

S/MIME

Handles S/MIME signed and/or encrypted mails. You can
decrypt mails, verify signatures or sign and encrypt your
own mails.

SpamAssassin

Enables the scanning of incoming mail received from a POP,
IMAP, or LOCAL
account using SpamAssassin. It can optionally delete mail
identified as spam or save it to a designated folder, and also
can be used to train a local Spamassassin (or a remote one if
Spamassassin >= 3.1 is installed both locally and remotely).
SpamAssassin is available from
http://spamassassin.apache.org/

Trayicon

Places an icon in the system tray that indicates whether
you have any new mail. A tooltip also shows the current
new, unread and total number of messages, and a contextual menu
allows the most common operations.

More...

Additionally, several external plugins are available on the Plugins
page. They are not included in Claws Mail to minimize the dependencies and size of
the core software, but provide nice, extra functions.